Stabilization of heterocyclic nitrogen compounds



United States atent STABILIZATION OFIHETEROCYCLIC NITROGEN COMPOUNDS Robert E. Reusser, Bartlesville, and Anton M. Schnitzer,

Dewey, "Okla, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application January 17, 1955, 'Seria'l'No. 482,382

'22 Claims. (Cl. 260-290) This invention relates to the stabilization of vinylsubstituted heterocyclic compounds having a hetero nitrogen atom. In a further aspect, this invention relates to the stabilization of vinylpyridine compounds.

One of the major difiiculties encountered in the manufacture of vinylpyridines and other vinyl-substituted heterocyclic nitrogen compounds is the tendency of these vinyl compounds to polymerize at ambient temperatures and, particularly, when subjected to elevated temperatures. This is an important problem because these vinyl compounds are used as monomers in polymerization reactions and the presence of soluble and/ or insoluble polymers in the vinylpyridine monomers can prevent their use for this purpose. Obviously, the problem reaches its peak when such vinyl compounds are processed in the presence of heat, for example, in the distillation of these compounds.

Vinylpyridines can be prepared by the dehydrogenation of a corresponding alltylpyridine by a method disclosed in the copending application of John E. Mahan, Serial No. 244,469, filed August 30, 1951, for The Preparation of Z-Met'hyl-S-Vinylpyridine From 2-Methyl-5-Ethylpyridine. The eflluent from this dehydrogenation reaction is separated in a combined steam and vacuum fractional distillation and this separation has been found to be very difficult, not only due to the proximity of the boiling points of the major components, but also due to the tendency of the Z-methyl-S-vinylpyridine to polymerize. The efiluent is subjected to heat in the range of 180 to 220 F. during this steam and vacuum distillation and operation of this distillation step can become impossible because of the formation of polymers unless a suitable stabilizing agent is used.

Polymers are also formed at ambient temperatures when the vinylpyridine compounds are stored and transported unless an efiective stabilizing agent is added.

It is an object of this invention, therefore, to provide a method for inhibiting the polymerization of vinyl-substituted heterocyclic compounds having a hetero nitrogen atom. It is a further object to provide a method for inhibiting the formation of both soluble and insoluble polymers in said vinyl-substituted heterocyclic compounds. It is a still further object to provide a method for preventing polymerization under conditions of fractional distillation of vinylpyridines. Other and further objects will be apparent from the following description.

We have discovered that the foregoing objects are attained by adding to vinyl-substituted heterocyclic compounds having a hetero nitrogen atom a polymerization inhibiting amount of certain azine dyes. The azine dyes which are efiective inhibitors in accordance with this '2 invention are, broadly, .theamino-substitutedphenylphenazinium chlorides represented by the .generalzformula wherein R can be a hydrogen, phenyl, or alkyl radical, said alkyl radical having up to and including '5 carbon atoms, and X can be a hydrogen, phenyl or said alkyl radical or wherein R is defined as stated above.

The azine-dyes to which this inventionrelates can also be defined as being selected fromthe group 'consistingof diamino phenylphenazinium chlorides, triaminoaphen'ylphenazinium chlorides, tetraamino-phenylphenazinium chlorides and phenyland alkyl-substituted derivatives thereof, wherein said alkyl 'substituents can have up to and including 5 carbon atoms.

Specific working examples of these azine dyes which can be used as inhibitors in accordance with our invention are '5-phenyl-3,7-diaminophenazinium chloride(phenosafranin); 5-phenyl-3-amino-7-dimethylaminophenazinium chloride (methylene violet); S-p'henyI-LS-dimethYl- 3,7diaminophenazinium chloride (safranin); 5-phenyl-2- phenylamino 3,7 di(phenylamino')phenazinium chloride (induline 3B); 5-phenyl-2,3,7,8-tetra'(phenylamino)phenachloride (induline 6B); 5-phenyl-3,7-di(diethylamino)phenazinium chloride (amethyst violet); 5'-phenyl- 3 dimethylamino-7-amino-S-methylphenazinium chloride (-Rhodulin violet B); 5-phenyl-3-phenylamino-7 amino- S-methyI henaZ-inium chloride (Methylene heliotrope); 5-phenyl-2,8-dimethyl-3-methylamino-7-ethylaminophenazinium chloride (Brilliant rhodulin red); 5-p'henyl-2,3-di- (phenylamino)-7-aminophenazinium chloride '(Inda'min blue .15); S-phcnyl-Zethyl-3,7-diamino-8-propylphenazinllIIIl chloride; S-phenyl-Z(N-phenyl-N propylamino) -3 (N'- ethyl-N'-pentylamino) -7-amino-8-butylaminophenazinium chloride; 5-phenyl-2,8-dipentyl 3,7 diaminophenazinium chloride; 5-phenyl- 2 (N-phenyl-N-methyl)-3,7,8 tri(diphenylamino)phenaz-inium chloride; 5-phenyl-2,8-dibutyl- 3,7-diaminophenazinium chloride; 5-phenyl-2,3;7,8 tetra- (dipentylamino) phenazinium chloride; and the. like. Structural formulas and the naming system followed herein for these azine dyes can be found in Encyclopedia of Chemical Technology by Raymond E. Kirk and Donald F. Othmer, volume 2, pages 214-223, published by The Interscience Encyclopedia, Inc., New York (1943;) and in Farbstofitabellen von Gustav Schultz, siebente auflage, :neu bearbeitet and erweitert vonDr. Ludwig Lehmann, Leipsiz, Akademische'Verlagsgesellschaft, M. .B. H. (1931), lithographed by Edwards Brothers, vInc, Ann Arbor, Michigan (1945).

As has been indicated above, these azine dyes are efice. g., over 50 per cent.

tive to inhibit the formationof both soluble and insoluble polymers in vinyl-substituted heterocyclic compounds having a hetero nitrogen atom. The insoluble polymer is also commonly known as popcorn polymer and is an insoluble, hard, porous and opaque material. Popcorn polymer is aptly named because it resembles popcorn in appearance.

We wish to point out that we intend that the terms stabilizer, inhibitor or inhibiting agent, used herein, shall have essentially the same meaning and that they are used for essentially the same purpose, which is to designate the substances disclosed herein which prevent polymerization of the vinyl-substituted heterocyclic nitrogen compounds. It is not intended that these terms necessarily shall denote a mechanism by which this stabilizing action is accomplished.

One group of polymcrizable vinyl-substituted heterocyclic compounds containing a hetero nitrogen atom which can be stabilized in accordance with our invention is the monoand divinylpyridines, with the vinyl radical being present in any of the several positions in the pyridine nucleus. Alkyl radicals can be present on the ring, or on the alpha carbon atom of the vinyl radical, but the number of carbon atoms in the combined alkyl radicals should generally not be greater than 12. These alkyl radicals are preferably methyl and ethyl radicals. Substituents attached to carbon atoms in the ring can be selected from the group consisting of hydrogen, alkyl,

vinyl and isopropenyl (alpha-methylvinyl) radicals; at

least one, and not more than two, of said radicals being vinyl or alpha-methylvinyl with the total number of carbon atoms in the alkyl radicals being not greater than 12. Examples of such compounds are 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine, 2,5-divinylpyridine, Z-methyl-S- vinylpyridine, 2-vinyl-5-ethylpyridine, 2,3,4-trimethyl-5- vinylpyridine, 3,4,5,6-tetramethyl-2-vinylpyridine, 3-ethyl- S-vinylpyridine, 2,6-diethyl-4-vinylpyridine, 2-isopropyl- 4-nonyl-5-viny1pyridine, Z-methyl-S-undecyl-4-vinylpyridine, 3-dodccyl-2,4-divinylpyridine, 2,3-dimethyl-5,6 dipentyl 4 vinylpyridine, 2 methyl 5 (alpha methylvinyl)pyridine, 2,5 -di(alpha-methylvinyl)pyridine, and the like.

Other polymerizable heterocyclic compounds containing a hetero nitrogen atom included within the scope of this invention are those vinyland alpha-methylvinyl-substituted heterocyclic nitrogen compounds in which the ring structure isunsaturated, partially saturated and completely saturated. Examples include vinyland alphamethylvinyl-substituted quinolines, isoquinolines, piperi-.

dines (hexahydropyridines), pyrroles, pyrrolidines, pyr- -rolidones, alkyl derivatives of the foregoing compounds,

dihydro and tetrahydropyridines, partially hydrogenated quinolines and isoquinolines, and pyrrolines (dihydropyrroles). xamples of such compounds are 2-vinylquinoline, 8-ethyl-2 vinylquinoline, 4-hexyl-5-vinylquinoline, 1- vinylisoquinoline, 5 methyl l isopropenylisoquinoline, 1,8 divinylisoquinoline, vinylpyrrolidone, vinylpyrrole, vinylpiperidine and vinylpyrrolidine, and the like. Normally, the vinyl substituent will be attached to a ring carbon atom. However, in compounds wherein the ring nitrogen atom is a secondary nitrogen atom, the vinyl group can instead be attached to this ring nitrogen atom,

for example, N-vinylcarbazole and N-vinylpyrrolidone.

. The process of this invention is particularly applicable to organic or hydrocarbon mixtures containing substantial proportions of a vinylpyridine compound, i. e., that amount which causes difiiculties due to its polymerization,

Examples of vinylpyridine compounds contained in such mixtures are 2-vinyl-4,6-dimeth- -ylpyridine, 2-methy1-4-vinylpyridine, Z-methyl-S-vinylpyridine, S-ethyl-Z-vinylpyridine, and 5-amyl-2-vinylpyridine. In carrying out the invention, solution of the stabilizing agent in the vinylpyridine, or other heterocyclic compound containing a hetero nitrogen atom to be stabilizeipcan be k as inhibitors of soluble polymer formation by the follow- 7 ing method. Weighed samples of V dine containing 5 weight per cent water were refluxed for accomplished in any suitable manner, such as by agitating or stirring the mixture containing the stabilizing agent, or merely by allowing the mixture to stand. Ultimately, the decision on what concentration of the stabilizing agent to use will depend on variable factors, such as the temperature and duration of time at a given temperature at which the heterocyclic compound containing a hetero nitrogen atom will be maintained when inhibition of polymer formation is desired, and economic considerations. In practice, a range of concentration from about 0.05 per cent to 2.0 per cent, or more, by weight of the azine compounds disclosed herein Will be employed, preferably from 0.1 per cent to 1.0 per cent, all percentages being based on the weight of the heterocyclic compound containing a hetero nitrogen atom to be stabilized. Furthermore, the amount will depend on which particular heterocyclic compound containing a hetero nitrogen atom is concerned.

EXAMPLE I Certain azine dyes within the scope of this invention were tested for their effectiveness as inhibitors of soluble polymer formation in 2-methyl5-vinylpyridine. Samples of Z-methyI-S-Vinylpyridine containing 5 weight per cent water were placed in vials and to each vial was added a small piece of steel and the inhibitor to be tested. The

vials were then flushed with nitrogen and sealed tightly with screw-on tops. The vials were then placed in an oven and maintained at 185 F. for 16 hours, after which a weighed sample of the contents of each vial was evaporated under vacuum and over boiling water. The residue which remained, with allowance for the weight of inhibitor present, was considered to be soluble polymer. The results of these tests are given below in Table I.

Table 1 Amount 01' Amount of In- Soluble Inhibitor hibitor (Weight Polymer Percent) Formed (Weight Percent) 0. 1. 40 Phenosafmnin 0. 0. 75 0. 0. 75 0. V 1. 60 Indullne 3B 0. 0. 0. 0. 72 0. 1. 91 Sair n {0. 1. 85 0. 1. 64 0. 1. 51 Methylene Violet- 0. 0. 79 0. 6 0. 70 Control (N 0 inhibitor) 25 The data above in Table I show that the azine dyes are very efiective to prevent the formation of soluble polymer in a vinyl-substituted heterocyclic nitrogen compound. This result is noteworthy because of the elevated temperature to which the samples were exposed and also because of the very small amount of polymer which was formed under these conditions. The control was simply a sample of 2-methyl-5-vinylpyridine which was treated in the way described for the other tests except that no inhibitor was added.

EXAMPLE H The azine dyes were also tested for their etfectiveness -methyl-5-vinylpyri- 16 hours at F. under conditions of total reflux. A glass column, two feet long and three-quarters of an inch 7 inside diameter was used. A small piece of wood having two inches of one-sixteenth inch diameter steel Wire wrapped around it was used as a boiling chip. The pressure was adjusted to keep the temperature of the distillation at 185 F. and this pressure was calculated to be approximately 250 millimeters of mercury. The refluxing procedures were conducted in the presence of varying amounts of certain azine dyes and after distillation, the sample was evaporated under vacuum over boiling water. The residue which remained, with allowance for the weight of inhibitor present, was considered to be soluble polymer. The control which is noted at the bottom of the following Table II was a sample of 2-methyl=S-vinylpyridine which was subjected to the same conditions as those samples containing an inhibitor except that no inhibitor was used. The results of these tests are tabulated below in Table H.

Table 11 Amount of Soluble Polymer Formed (Weight Percent) Amount of Inhibitor Used (Weight Percent) Inhibitor Phenosatranin 0.3-- dullne 33 0.3. Safranin 3 Methylene Violet 0.3- Control (No Inhibitor)" The information above in Table II is particularly important because it demonstrates that these inhibitors are eifective in distillation processes where the need for these inhibitors is of the greatest commercial importance.

EXAMPLE HI were prepared in vials as is described above in Example I,

these samples containing about 5 per cent water. To each sample was added 0.3 weight per cent divinylbenzene and the stated amount of the inhibitor. The inhibitor was added to the sample after the sample has been allowed to stand for about 5 hours at 185 F. This five hour period of incubation produced a small seed of popcorn polymer after which the inhibitors were added to the sample. The control was prepared in the same way except that no inhibitor was placed in this sample. The vials were placed in an oven maintained at 185 F. andthe results of these tests are noted below in Table .111.

Table III Amount of Inhibitor Used (Weight Percent) Inhibitor Results Phenosairanimuu Methylene Violet- Induline 3B 0.1 Control (No inhibitor) NopDopcorn polymer in '20 days.

Do. 100 percent popcornpolymer inS hours.

The data above in Table III shows that the azine dyes are remarkably effective to prevent the formation of insoluble or popcorn polymer even when the sample of the vinyl-substituted heterocyclic nitrogen compound was allowed to incubate at an elevated temperature and to form a seed of popcorn polymer before the inhibitor was added.

EXAMPLE IV Is pared inthe same way except that noinhibitor was added. The results of these tests are reported'below in Table IV.

The conditions of these tests are very severe for testing inhibitors of vinylpyridines. It will be noted that the temperature was elevated to that at which the Z-methyl- S-vinylpyridine issubjected in a distillation process and the seeding of the samples generally produces the conditions under which popcorn polymer is most easily formed. Even under these conditions, it will be noted that the azine dyes were remarkably reflective to prevent the formation of insoluble polymer.

EXAMPLE V Further tests of the azine dyes as polymerization in- .hibitors were made to determine their eitectiveness under conditions of storage and/ or transit of 2-methyl-5-vinylpyridine. In these tests, the vials, each containing a small piece of steel, were charged with flashed Z-methyl-S-vinylpyridine to which had been added 0.1 weight per cent of the inhibitor. 3.5 per cent of the volume of each vial wasleft empty to provide an air cover for each sample and :to thereby simulate conditions of storage and transit. The vials were sealed with .screw .caps and maintained at F. during the test period.- The results of these tests are given below in Table V.

The data in Table V shows that only a very small amount of soluble polymer was formed in the Z-methyl- S-vinylpyridine under the conditions of the test. The amount of soluble polymer reported in Table V was determined by the hexane precipitation method, in which the polymer is precipitated out in n-hexane, filtered, dried and weighed.

EXAMPLE VI Tests were made to determine the effectiveness of these azine dyes as inhibitors during the vacuum distillation of Z-methyl-S-vinylpyridine in a pilot plant column which was controlled so as to closely simulate commercial scale operations wherein the effiuent from the dehydrogenation of Z-methyl-S-ethylpyridine is subjected to a steam and vacuum distillation. Z-Methyl-S-vinylpyridine, .to which had been added 0.03 weight per cent of phenosafranin, was fed into the top of the column. The phenosafranin was added to Z-methyl-S-vinylpyridine in the column kettle, initially, to give a concentration of 0.3 weight per cent of phenosafranin, which concentration was maintained by proper adjustment of the overheadtakeoif and the feed rate of the afore-described mixture of 2-methylvacuum and overboiling water.

-vinylpyridine and 0.03 weight per cent phenosafranin at the top of the column. The distillation was carried on for hours at a pressure of 225 millimeters of mercury.

for polymer formation by the method described in Example I, i. e., a weighed sample Was evaporated under The residue which remained, with allowance for the weight of inhibitor present, was considered to be soluble polymer. The results of these runs and the samples tested are given below in Table VI.

Table VI Weight percent soluble polymer 7 hours 8hours 9hours lOhours Run A Run B It-is clear from these tests that these azine dyes are very eifective to prevent polymer formation in a steam and vacuum distillation of a mixture of vinylpyridines such as is produced by the dehydrogenation of 2-methy1-5- ethylpyridine to produce Z-methyl-S-vinylpyridine.

The azine dyes each have their own characteristic color. It has been found, however, that color produced in the polymerizable materials by adding the dye is only the color of the dye itself and it has been found that this 1 color and the azine dyes themselves can be easily and effectively removed from the vinyl-substituted pyridines by a simple flash distillation. As was indicated above, the vinylpyridines are useful as monomers in a variety of reactions and in many cases, it is desirable that the vinyldistillation column, any point along the column or the inhibitors can be added to the reflux of the distillation column. It will be found that in some cases, a smaller amount of the inhibitor is eifective when the inhibitor is added to the top of the column or to the column reflux.

it will be understood that the foregoing disclosure is illustrative of our invention and that other embodiments within the scope of the invention will occur to those skilled in the art. Those skilled in the art will appreciate, for instance, that for the purpose of this invention, vinyl-substituted heterocyclic compounds having a hetero nitrogen atom further substituted with non-interfering groups, for example, halo-, nitro-, amino-, hydroXyl-, and carboxyl-, are the same as non-substituted vinyl heterocyclic compounds having a hetero nitrogen atom.

We claim:

1. A method for inhibiting the polymerization of a heterocyclic nitrogen compound selected from the group consisting of compounds having the general formula (C C HIM wherein n is an integer selected from the group consisting of 1 and 2, each R is individually selected from the group consisting of H, CH3 and C2H5, and each R is individually selected from the group consisting of H and an a kyl group, not more than 12 carbon atoms being present in the total of said R groups, which comprises adding to said heterocyclic compound a polymerization inhibiting amount of an inhibitor according to the formula wherein R is a member selected from the group consisting of hydrogen, phenyl and alkyl radicals having up to 5 carbon atoms and X is a member selected from the group consisting of hydrogen, phenyl, alkyl radicals and wherein said alkyl radicals and R are defined as hereinbefore stated.

2. A method according to claim 1 wherein said heterocyclic compound is a vinyl-substituted pyridine.

3. A method according to claim 1 wherein said heterocyclic compound is 2-methyl-5-vinylpyridine.

4. A method according to claim 1 wherein said inhibitor is phenosafranin.

5. A method according to claim 1 wherein said inhibitor is methylene violet.

6. A method according to claim 1 wherein said in hibitor is safranin.

7. A method according to claim 1 wherein said inhibitor is induline 3B.

8. A method according to claim 1 wherein said inhibitor is induline 6B.

9. A method for stabilizing a vinylpyridine against polymerization which comprises adding from 0.05 weight per cent to 2.0 weight per cent based on said vinylpyridine of an inhibitor according to the formula wherein R is a member selected from the group consisting of hydrogen, phenyl and alkyl radicals'having up to 5 carbon atoms and X is a member selected from the group consisting of hydrogen, phenyl, alkyl radicals and wherein said alkyl radicals and R are defined as hereinbefore stated.

10. A method according to claim 9 wherein the amount of said inhibitor is from 0.1 weight per cent to 1.0 weight per cent.

11. A method according to claim wherein said inhibitor is phenosafranin.

12. A method according to inhibitor is methylene violet.

13. A method according to inhibitor is safranin.

14. A method according to inhibitor is induline 3B.

15. A method according to inhibitor is induline 6B.

16. In a process for preparing Z-methyl-S-vinylpyridine comprising the dehydrogenation of 2-methyl-5-ethylpyridine and the fractional distillation of the efiluent from said dehydrogenation wherein said efliuent is subjected to heat in the range from 180 to 220 F., a method for inhibiting polymerization of said efiluent during said distillation which comprises adding to said eflluent a polymerization inhibiting amount of an inhibitor according to the formula claim 10 wherein said claim 10 wherein said claim 10 wherein said claim 10 wherein said wherein R is a member selected from the group consisting of hydrogen, phenyl and alkyl radicals having up to 5 carbon atoms and X is a member selected from the group consisting of hydrogen, phenyl, alkyl radicals and wherein n is an integer selected from the group consisting of 1 and 2, each R" is individually selected from the group consisting of H, CH3 and Cal-I5, and each R is individually selected from the group consisting of H and an alkyl group, not more than 12 carbon atoms being present in the total of said R groups, said heterocyclic base being stabilized by the addition of a stabilizing amount of a compound selected from the group consistwherein R is a member selected from the group consisting of hydrogen, phenyl and alkyl radicals having up to 5 carbon atoms and X is a member selected from the group consisting of hydrogen, phenyl, alkyl radicals and wherein said alkyl radicals and R are defined as hereinbefore stated.

18. A composition of matter in accordance with claim 17 wherein said inhibitor is phenosafranin.

19. A composition of matter in accordance with claim 17 wherein said inhibitor is methylene violet.

20. A composition of matter in accordance with claim 17 wherein said inhibitor is safranin.

21. A composition of matter in accordance with claim 17 wherein said inhibitor is induline 3B.

22. A composition of matter in accordance with claim 17 wherein said inhibitor is induline 6B.

OTHER REFERENCES Dunbrook et al.: Official Gazette, vol. 644, pp. 623-4.

Frank et al.: J. Am. Chem. Soc., vol. 68, p. 908 (1946).

Rac: Chem. Abst., vol. 40, cols. 4895-6 (1946). 

1. A METHOD FOR INHIBITING THE POLYMERIZATION OF A HETEROCYCLIC NITROGEN COMPOUND SELECTED FROM THE GROUP CONSISTING OF COMPOUNDS HAVING THE GENERAL FORMULA 